I. Field of the Invention
The present invention relates in general to microwave balloon angioplasty, and pertains more particularly to a microwave or radiofrequency catheter system for the heating of plaque in arteries or blood vessels. Also described herein are improvements pertaining to features of the microwave catheter system, including improved antenna constructions and associated fiberoptics.
II. Background Discussion
Balloon angioplasty is now a relatively well-accepted alternative to bypass surgery for high-grade obstructive atherosclerotic lesions of the peripheral, renal and coronary vessels. In this regard, U.S. Pat. No. 4,643,186, entitled "Percutaneous Transluminal Microwave Catheter Angioplasty," by Rosen et al., describes a coaxial cable and antenna for microwave heating of artery plaque. This system suffers from several shortcomings which make it difficult, if not impossible, to develop a well controlled volume of heat within the plaque material. Also, for small arteries where catheter diameter and flexibility are critical, the system described by Rosen et al. does not allow for sufficient transmission of microwave power to the plaque for welding purposes. Recent work with laser balloon angioplasty demonstrates the need to heat the plaque to nominally 100.degree. C. for 30 seconds. For a 1.34 m. length of commercial microwave coax, the insertion loss at 10GHz is approximately 10 dB. This frequency corresponds to a depth of penetration in plaque of 3 mm. Therefore, a 35 watt load requirement for heating plaque to nominally 100.degree. C. requires a 350 watt power supply (RF). This situation is not practical. The transmission line itself would heat up, because 315 watts is dissipated by it during power transmission to the plaque (load). The monopole antenna described in Rosen et al. does not provide radiation confined solely to the distal end inside the balloon. A very nonuniform radiation pattern is developed with antenna current leaking back up the outside surface of the outer conductor, which forms the coax. The resulting heating pattern is sharply peaked at the point along the coax where the inner conductor protrudes outside of the outer conductor and a secondary heating pattern develops along the length of the coax back to the generator. Leakage currents produce the secondary heating pattern. This may result in melting of the catheter plastic material.
Accordingly, it is an object of the present invention to provide an improved technique for the heating of plaque in arteries, veins or blood vessels, such as in association with microwave balloon angioplasty.